For articles made of polypropylene, there has been demanded good balance between excellence in transparency, impact resistance, and mechanical properties (flexibility or rigidity, etc.) and absence of deterioration in these properties at high temperatures (hereinafter, may be called “heat resistance”). So far, there has never existed a material with these properties in a well-balanced manner.
For example, articles made of polypropylene are excellent in rigidity, heat resistance, surface gloss, scratch resistance, and the like, and hence they are widely used. However, there has been a problem that their poor low-temperature impact resistance imposes limitation on the application. As a measure to improve low-temperature impact resistance of such polypropylene, there has been widely known block PP, which is produced by sequential polymerizations to yield polypropylene moiety and ethylene/propylene copolymer moiety. Such block PP has rigidity, excellent heat resistance, which means retention of the shape even at high temperatures, and improved low-temperature impact resistance, but its transparency is unsatisfactory.
On the other hand, it is known that propylene/α-olefin copolymer, so-called random PP, in which propylene is copolymerized with at least one or more α-olefins having 2 to 20 carbon atoms (excluding propylene), is inferior in low-temperature impact resistance, although random PP has improved transparency by adding a nucleating agent or the like.
Food containers such as containers for frozen storage are required to superior heat resistance such as the shape retention in heating with a microwave oven as well as transparency, rigidity, and low-temperature impact resistance in frozen storage.
Retort pouches used for retort foods and the like are required to have excellent transparency, low-temperature impact resistance, and flexibility, and at the same time, excellent heat resistance such that those properties are not deteriorated even in sterilization at high temperatures.
Generally, polyethylene and propylene random copolymer are used as polymers to form industrial materials such as food containers and as polymers to form packaging materials such as retort pouches.
Various proposals for propylene random copolymer have already been made. For example, there has been proposed a propylene-based ternary copolymer obtained by copolymerizing propylene with a small amount of ethylene and butene (see Patent Documents 1, 2, and 3). The propylene-based ternary copolymer is crystalline and excellent in heat resistance, but it is insufficient in balance between transparency and low-temperature impact resistance for using as an industrial material and insufficient in flexibility for using as a packaging material.
There has been proposed a method for producing a propylene-based ternary copolymer capable forming films that is excellent in rigidity, transparency, and easiness to open and also superior in heat-sealing properties. This copolymer contains 1.3 to 2.4 wt % of ethylene units and 6.5 to 12.1 wt % of butene units together with propylene units (Patent Document 4).
As a propylene-based ternary copolymer excellent in low-temperature heat-sealing properties, there has been proposed a propylene-based ternary copolymer that is obtained by solution polymerization and contains 1 mol % or more of ethylene units, 1 mol % or more of butene units, and less than 90 mol % of propylene units (Patent Document 5).
It is proposed that a propylene-based ternary copolymer consisting of 80 to 96.5 wt % of propylene units, 3 to 17 wt % of ethylene units, and 0.5 to 5 wt % of butene units is formed into injection-molded rigid product or blow-molded bottles (Patent Document 6).
It was found that, however, food containers, retort pouches, and the like obtained from conventional polyethylene or propylene-based ternary copolymer described above are sometimes whitened to decrease transparency or sometimes become less flexible when heated at temperatures high enough for sterilization for a given time.
The present applicants have already proposed, for example, a polyolefin composition that obtained from 50 to 95 parts by weigh of propylene-based polymer (A) containing 0 to 10 mol % of ethylene units (a), 100 to 80 mol % of propylene units (b), and 0 to 15 mol % of units (c) of an α-olefin having 4 to 12 carbon atoms; and 5 to 50 parts by weight of propylene-based random copolymer (B) containing 2 to 20 mol % of ethylene units (a), 80 to 30 mol % of propylene units (b), and 10 to 50 mol % of units (c) of an α-olefin having 4 to 12 carbon atoms (Patent Document 7). However, according to the study of the present applicants, it was found that there was room for improvement in balance among transparency, low-temperature impact resistance, and heat resistance.
Propylene/ethylene/1-butene copolymer was also used in Patent Document 8, but used as a thermoplastic elastomer, and it was found that heat resistance and low-temperature impact strength thereof were still to be increased.
Patent Document 9 discloses a block copolymer composed of polypropylene block and propylene/ethylene/1-butene copolymer block, which was also used as an elastomer, and it was found that room still remained for improvement in the heat resistance and low-temperature impact strength.    [Patent Document 1] Japanese Patent Laid-Open Publication No. S51-79195    [Patent Document 2] Japanese Patent Laid-Open Publication No. S53-26882    [Patent Document 3] Japanese Patent Laid-Open Publication No. S53-26883    [Patent Document 4] Japanese Patent Application Publication No. S55-6643    [Patent Document 5] Japanese Patent Laid-Open Publication No. S57-125207    [Patent Document 6] Japanese Patent Laid-Open Publication No. S57-147506    [Patent Document 7] Japanese Patent Laid-Open Publication No. H08-283491    [Patent Document 8] Japanese Patent Laid-Open Publication No. H08-301934    [Patent Document 9] Japanese Patent Laid-Open Publication No. 2001-064335